SEOUL (Reuters) - It may have taken the collapse of an apartment block in an exclusive district of the North Korean capital to reveal the Achilles heel of young leader Kim Jong Un's secretive regime.

Last week's accident killed the families of people important enough for North Korea to issue an obsequious and unprecedented public apology in a bid to quell public anger, some analysts said.

The 23-storey building in Phyongchon, central Pyongyang, was part of a construction boom driven by Kim that includes apartment blocks, roads, bridges and the Masik Ski Resort that has become synonymous with his policy of finishing projects at lightning speed.

South Korea said the building was home to 92 families and hundreds are feared dead, although the May 13 accident happened in the afternoon and many residents would have been at school or at work..

Apartments in buildings taller than 20 stories are normally reserved for party officials, professionals, academics and managers at state agencies, and those recognised for contribution to the state, the Architecture Institute of (South) Korea said last year.

"They would be the kind of people who the state can't just choose to ignore and act like nothing happened," a South Korean official with access to intelligence on the North said.

They are not the highest ranks of the North's ruling class but they are "people who have power or money", said Kwak In-su, a North Korean defector who works at the Institute for National Security Strategy, run by the South's spy agency.

"People have mobile phones and talk," Kwak said. "Angry public opinion can become a boomerang so the regime wants to prevent it early."

The collapse follows a period of political upheaval and intrigue and raises questions about the safety of other buildings going up in the same area.

ACCIDENTS "HAPPEN ALL THE TIME"

Kim, who rose to power when his father died in December 2011, has removed most of Pyongyang's old guard during his comparatively short rule, replacing ageing generals and cadres.

He has changed his Korean People's Army chief of staff four times. Kim's uncle, Jang Song Thaek, considered the second most powerful man in North Korea, was executed in December.

The scale of Kim's construction projects had puzzled outside analysts as to how the impoverished state was able to supply the materials and equipment needed, and some suggested the North may be doing without, or using fewer, crucial materials like steel.

Accidents like last week's collapse "happen all the time", the South Korean official said, adding only that the location was different. Phyongchon is within walking distance of Kim's office.

"This case is unusual in that unlike most apartment blocks, for working people for example, they would have used better materials ... and followed (safety) standards, and still this happened," he said.

Access to North Korea is severely restricted, but satellite images show the collapsed building as having had balconies. As is the case in South Korea, this is where families would store their kimchi pots through the winter, or put out fake flowers during festivals celebrating the birthdays of the Kim dynasty if facing the road.

There are 17 or more high-rise buildings that are new or under construction in the same area, Curtis Melvin, of the U.S.-Korea Institute at the Johns Hopkins University School of Advanced International Studies, said.

An undercover article published by Asia Press International in 2011 showed pictures of buildings under construction in Pyongyang, reporting a shortage of building materials and food for the workers.

One picture showed an apartment block of more than 20 storeys with the location and size of windows changing slightly from floor to floor.

Many of the buildings going up also do not have elevators, a Reuters witness said, because of the frequent power outages.

"The collapsed building was neither the first finished nor the most prestigious," said Melvin, who analyses satellite imagery. "This raises serious questions about the safety of the other buildings."

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